Thermal and mechanical properties of silane‐grafted water crosslinked polyethylene

The effects of linear low density polyethylene (LLDPE) grafting with vinyltrimethoxysilane by different types and contents of peroxide were studied. When grafting silane onto LLDPE, with 0.10 phr of Dicumyl peroxide (DCP) or 0.05 phr content of 2,5‐Dimethyl‐2,5‐di (tert‐butyl‐peroxy)‐hexane (DHBP), it was found that the grafting effect was improved; however, as Di(2‐tert‐butylperoxypropyl ‐(2))‐benzene (DIPP) or excess DHBP was used, LLDPE was supposed to cause self‐crosslinking, which reduced the grafting effect of silane and was invalid in the processing of extrusion. In this study, vinyl trimethoxysilane (VTMS) was grafted onto various polyethylenes (HDPE, LLDPE, and LDPE) using DCP as an initiator in a twin screw extruder. The grafted polyethylenes were able to crosslink utilizing water as the crosslinking agent. The effects of varied crosslinking time on the mechanical properties of the crosslinked polyethylenes were studied. It was found that the HDPE and LLDPE were apt to crosslink during the grafting process and thus decreased the grafting ratio. Multiple melting behavior was observed for crosslinked LDPE and LLDPE. Mechanical and thermal properties of the crosslinked PE are much better than that of uncrosslinked PE. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2383–2391, 2005     

Mechanical properties and morphology of polyethylene–polypropylene blends with controlled thermal history

The effect of time–temperature treatment on the mechanical properties and morphology of polyethylene–polypropylene (PE–PP) blends was studied to establish a relationship among the thermal treatment, morphology, and mechanical properties. The experimental techniques used were polarized optical microscopy with hot‐stage, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and tensile testing. A PP homopolymer was used to blend with various PEs, including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low density polyethylene (VLDPE). All the blends were made at a ratio of PE:PP = 80:20. Thermal treatment was carried out at temperatures between the crystallization temperatures of PP and PEs to allow PP to crystallize first from the blends. A very diffuse PP spherulite morphology in the PE matrix was formed in partially miscible blends of LLDPE–PP even though PP was present at only 20% by mass. Droplet‐matrix structures were developed in other blends with PP as dispersed domains in a continuous PE matrix. The SEM images displayed a fibrillar structure of PP spherulite in the LLDPE–PP blends and large droplets of PP in the HDPE–PP blend. The DSC results showed that the crystallinity of PP was increased in thermally treated samples. This special time–temperature treatment improved tensile properties for all PE–PP blends by improving the adhesion between PP and PE and increasing the overall crystallinity. In particular, in the LLDPE–PP blends, tensile properties were improved enormously because of a greater increase in the interfacial adhesion induced by the diffuse spherulite and fibrillar structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1151–1164, 2000     

Silane grafting and moisture crosslinking of polyethylene: The effect of molecular structure

The silane grafting and moisture crosslinking of different grades of polyethylene have been investigated. Three types of polyethylene (HDPE, LLDPE, and LDPE) with different molecular structures and similar melt flow indices were selected. The initiator was dicumyl peroxide (DCP), and the silane was vinyltrimethoxysilane. The grafting reaction was carried out in an internal mixer. The extent of grafting and the degree of crosslinking were determined, and hot‐set tests were carried out to evaluate the crosslink structure of the different polyethylenes. The LLDPE had the highest degree of grafting, while the LDPE had the least. The rate of crosslinking for LDPE was higher than that of HDPE and LLDPE. The gel content of LDPE was higher than that of HDPE and LLDPE. Hot‐set elongation and the number‐average molecular weight between crosslinks (M_c) were lower for LLDPE and LDPE than for HDPE. Increasing the silane/DCP percentage led to peroxide crosslinking, thereby decreasing the M_c and hot‐set elongation. The number‐average molecular weight (M_n), molecular weight distribution, and number of chain branches were the most important parameters affecting the silane grafting and moisture crosslinking. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Rheological and mechanical properties in polyethylene blends

Melt rheology and mechanical properties in linear low density polyethylene (LLDPE)/low density polyethylene (LDPE), LLDPE/high density polyethylene (HDPE), and HDPE/LDPE blends were investigated. All three blends were miscible in the melt, but the LLDPE/LDPE and HDPE/LDPE blends exibiled two crystallization and melting temperatures, indicating that those blends phase separated upon cooling from the melt. The melt strength of the blends increased with increasing molecular weight of the LDPE that was used. The mechanical properties of the LLDPE/LDPE blend were higher than claculated from a simple rule of mixtures, whiele those of the LLDPE/HDPE blend conformed to the rule of mixtures, but the properties of HDPE/LDPE were less than the rule of mixtures prediction.     

Co‐crystallization in ternary polyethylene blends: tie crystal formation and mechanical properties improvement

Understanding the co‐crystallization behavior of ternary polyethylene (PE) blends is a challenging task. Herein, in addition to co‐crystallization behavior, the rheological and mechanical properties of melt compounded high density polyethylene (HDPE)/low density polyethylene (LDPE)/Zeigler ? Natta linear low density polyethylene (ZN‐LLDPE) blends have been studied in detail. The HDPE content of the blends was kept constant at 40 wt% and the LDPE/ZN‐LLDPE ratio was varied from 0.5 to 2. Rheological measurements confirmed the melt miscibility of the entire blends. Study of the crystalline structure of the blends using DSC, wide angle X‐ray scattering, small angle X‐ray scattering and field emission SEM techniques revealed the formation of two distinct co‐crystals in the blends. Fine LDPE/ZN‐LLDPE co‐crystals, named tie crystals, dispersed within the amorphous gallery between the coarse HDPE/ZN‐LLDPE co‐crystals were characterized for the first time in this study. It is shown that the tie crystals strengthen the amorphous gallery and play a major role in the mechanical performance of the blend.© 2016 Society of Chemical Industry     

Dynamic vulcanization of recycled milk pouches (LDPE–LLDPE) and EPDM blends using dicumyl peroxide

The viability of thermomechanical recycling of post‐consumer milk pouches (blend of low‐density polyethylene (LDPE) and linear low‐density polyethylene (LLDPE)) and its scope for suitable engineering applications were investigated. The effects of blending with ethylene‐propylene‐diene monomer (EPDM) rubber and subsequent curing using dicumyl peroxide (DCP) on the macromolecular structure and properties of recycled polyethylene (PE) blends were studied. The crosslinking efficiency of recycled PE/EPDM blends and possible thermooxidative degradation of recycled polymer upon peroxide curing was assessed using torque and gel content measurements along with infrared spectroscopic analysis. Both the torque and gel content of the blends varied with DCP crosslinking reactions and also were affected by oxidative degradation. In view of the electrical application area of this recycled blend material, the dielectric breakdown strength and volume resistivity were measured. The mechanical performance and thermal stability of recycled PE/EPDM blends improved with progressive crosslinking by DCP but deteriorated somewhat at higher DCP dose. Scanning electron microscopy showed good interface bonding between recycled polymer and dispersed EPDM phase in the cured blends compared to the non‐cured blends. Copyright © 2007 Society of Chemical Industry     

不同分子链结构聚乙烯的交联研究

研究了线性、短支链和长支链支化三种分子链结构聚乙烯的化学交联过程,对交联体系的机械性能和热性能进行了分析对比。结果表明,在同样交联剂含量情况下,聚乙烯交联体系的凝胶含量顺序是:长支链支化>短支链支化>线性结构;交联引起的拉伸强度变化率为长支链支化>短支链支化>线性结构;交联引起的断裂伸长率变化率为线性结构>短支链支化>长支链支化;交联引起结晶度的下降程度为线性结构>短支链支化>长支链支化。     

Thermal and wide angle X‐ray analysis of chemically and radiation‐crosslinked low and high density polyethylenes

Low and high density polyethylenes (PE) were crosslinked by two methods, namely, chemically by use of different amounts of tert‐butyl cumyl peroxide (BCUP) and by irradiation with different doses of electron beam. A comparison between the effects of these two types of crosslinking on crystalline structure, crystallinity, crystallization, and melting behaviors of PE was made by wide angle X‐ray diffraction and DSC techniques. Analysis of the DSC first heating cycle revealed that the chemically induced crosslinking, which took place at melt state, hindered the crystallization process and decreased the degree of crystallinity, as well as the size of crystals. Although the radiation‐induced crosslinking, which took place at solid state, had no significant influence on crystalline region, rather, it only increased the melting temperature to some extent. However, during DSC cooling cycle, the crystallization temperature showed a prominent decrease with increasing irradiation dose. The wide angle X‐ray scattering analysis supported these findings. The crystallinity and crystallite size of chemically crosslinked PE decreased with increasing peroxide content, whereas the irradiation‐crosslinked PE did not show any change in these parameters. As compared with HDPE, LDPE was more prone to crosslinking (more gel content) owing to the presence of tertiary carbon atoms and branching as well as owing to its being more amorphous in nature. HDPE, with its higher crystalline content, showed relatively less tendency toward crosslinking especially by way of irradiation at solid state. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3264–3271, 2006     

Formation and characterization of polyethylene blends for autoclave‐based expanded‐bead foams

Blends of linear‐low‐density polyethylene (LLDPE), low‐density polyethylene (LDPE), and high‐ density polyethylene (HDPE) were foamed and characterized in this research. The goal was to generate clear dual peaks from the expanded polyethylene (EPE) foam beads made from these blends in autoclave processing. Three blends were prepared in a twin‐screw mixing extruder at two rotational speeds of 5 and 50 rpm: Blend1 (LLDPE with 20 wt% HDPE), Blend 2 (LLDPE with 20 wt% LDPE), and Blend 3 (LLDPE with 10 wt% HDPE and 10 wt% LDPE). The differential scanning calorimetric (DSC) measurement was taken at two cooling rates: 5 and 50°C/min. Although no dual peaks were present, the results showed that blending with HDPE has a more noticeable effect on the DSC curve of LLDPE than blending with LDPE. Also, the rotational speed and cooling rate affected the shape of the DSC curves and the percentage area below the onset point. The DSC characterization of the batch foamed blends revealed multiple peaks at certain temperatures, which may be mainly due to the annealing effect during the gas saturation process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry

Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 681–692, 2003     

交联聚乙烯绝缘料用基础树脂的性能研究

利用拉伸试验、红外光谱、凝胶渗透色谱、差示扫描量热法、流变表征等方法测试了交联聚乙烯绝缘料用基础树脂的性能,并研究了制备硅烷交联和过氧化物交联聚乙烯绝缘料的基础树脂组成。结果表明,交联聚乙烯绝缘料用LDPE树脂的基本性能:MFR为2.0g/10 min,密度为0.920g/cm3,熔点为107℃左右,拉伸强度大于12MPa,断裂伸长率大于580%,介电常数小于2.3,相对支化度2.34左右。硅烷交联聚乙烯绝缘料宜用LLDPE和LDPE的共混物做基础树脂,而过氧化物交联聚乙烯绝缘料的基础树脂用LDPE即可。     

硅烷接枝热水交联聚乙烯共混物的研究

本文介绍了采用过氧化二异丙苯（ ＤＣＰ） 为引发剂,二月桂酸二丁基锡（ ＤＢＴＬ） 为催化剂,在双螺杆挤出机中硅烷（ ＶＴＥＳ） 与聚乙烯（ＰＥ） 共混物的接枝反应,以及通过热水交联获得硅烷交联聚乙烯共混物。研究了不同牌号聚乙烯的选取,正交实验优化配方以及热水交联时间对产品性能的影响。结果表明：选取适当的高密度聚乙烯,低密度聚乙烯及线性低密度聚乙烯共混物进行硅烷接枝交联,可获得具有良好的力学性能和良好的加工性能的交联ＰＥ 共混物;正交实验获得ＤＣＰ、ＶＴＥＳ 及ＤＢＴＬ 的最佳用量分别为０ ．１０ 份、２ ．５ 份、０ ．１５ 份;热水交联的合适时间约为１２ 小时,在此之前拉伸强度、维卡软化点及凝胶率随交联时间而升高,断裂伸长率随时间而降低,此后,几者均趋于稳定。     

Effect of EVA copolymer on properties of different polyethylenes in silane crosslinking process

Abstract

The effect of molecular structure of polyethylene (PE) [low density PE (LDPE), linear LDPE and high density PE] and silane/peroxide concentration on the grafting level and gel content in silane crosslinking process has been studied. The effect of incorporation of ethylene vinyl acetate (EVA) copolymer on the rate of crosslinking and thermal properties of PEs has been reported. The order of gel content was LDPE>linear LDPE>high density PE. With the incorporation of EVA, the rate of crosslinking increased. The degree of crystallinity did not change with crosslinking significantly. However, the shape of melting and crystallisation peaks changed, and two regions due to gel and sol parts were formed. In EVA/PE blends, two melting points were observed for both crosslinked and uncrosslinked samples. The SEM images showed the droplet matrix morphology with the EVA as the dispersed phase, especially for EVA/LDPE blend. The EVA/PE blends failed in hot set test, while the origin of PEs passed the hot set test successfully.     

Crystallization kinetics modeling of high density and linear low density polyethylene resins

This paper describes isothermal and nonisothermal crystallization kinetics of a Ziegler‐Natta catalyzed high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins. Standard techniques such as differential scanning calorimetry (DSC) and light depolarization microscopy (LDM) techniques were used to measure isothermal kinetics at low supercoolings. DSC was also used to measure nonisothermal crystallization kinetics at low cooling rates. Extrapolation of isothermal crystallization half‐times of Z‐N catalyzed LLDPE resin using the isothermal half‐time analysis led to erroneous predictions, possibly due to Z‐N LLDPE consisting of a mixture of molecules having different amounts of short chain branching (comonomer). However, predicted reciprocal half‐times at high supercoolings, using isothermal half‐time analysis and using nonlinear regression of nonisothermal crystallization kinetics measured at low cooling rates using the differential Nakamura model, of the HDPE were similar to measured reciprocal half times at high supercoolings of a similar HDPE by Patki and Phillips. It is also shown that the differential Nakamura model can be effectively used to model nonisothermal crystallization kinetics of HDPE resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE

Different types of polyethylene blown films (HDPE, LDPE, LLDPE) differ significantly in the ratio between machine and transverse direction tear resistance. In this paper, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) blown films at different draw-down ratios are studied, and the relation between crystalline structure and anisotropy of blown film properties is investigated. The crystalline morphology and orientation of HDPE, LDPE, LLDPE blown films were probed using microscopy and infrared trichroism. Significant differences in crystalline morphology were found: at medium DDR HDPE developed a row-nucleated type morphology without lamellar twisting, LDPE showed rod-like crystalline morphology and turned out to the row-nucleated structure with twisted lamellae at high draw-down ratio (DDR), while a spherulite-like superstructure was observed for LLDPEs at all processing conditions. They also showed quite different orientation characteristics corresponding to different morphologies. The morphologies and orientation structure for LDPE, LLDPE and HDPE are related to the stress applied (DDR) and their relaxations in the flow-induced crystallization process, which determine the amount of fibrillar nuclei available at the time of crystallization and therefore, the final crystalline morphology. These structure differences are shown to translate into different ratios of machine and transverse direction tear and tensile strengths.     

Structure–property–processing relationships in chemically modified LDPE and LDPE/LLDPE blend

The properties of low-density polyethylene (LDPE) and its blend with linear low-density polyethylene LDPE : LLDPE (75 : 25) modified with varying concentrations of dicumyl peroxide (DCP) were compared. Chemical modification was carried out in a Brabender plasticorder under set conditions of temperature, rotor speed, and time. The results are reported in terms of the type of polyethylene used, concentration of DCP, mixing torque, temperature and time on the processing, and mechanical and physical properties of the modified samples. Interpretation of the results is given in terms of gel contents which relate to the three-dimensional network structure of polyethylene. The modified blend forms a denser network structure and, hence, better product properties relative to that of modified LDPE, indicating the influence of LLDPE in increasing the rate and extent of crosslinking on blending with LDPE. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:789–797, 1998     

Crystallization of low‐density polyethylene‐ and linear low‐density polyethylene‐rich blends

The crystallization of a series of low‐density polyethylene (LDPE)‐ and linear low‐density polyethylene (LLDPE)‐rich blends was examined using differential scanning calorimetry (DSC). DSC analysis after continuous slow cooling showed a broadening of the LLDPE melt peak and subsequent increase in the area of a second lower‐temperature peak with increasing concentration of LDPE. Melt endotherms following stepwise crystallization (thermal fractionation) detailed the effect of the addition of LDPE to LLDPE, showing a nonlinear broadening in the melting distribution of lamellae, across the temperature range 80–140°C, with increasing concentration of LDPE. An increase in the population of crystallites melting in the region between 110 and 120°C, a region where as a pure component LDPE does not melt, was observed. A decrease in the crystallite population over the temperature range where LDPE exhibits its primary melting peaks (90–110°C) was noted, indicating that a proportion of the lamellae in this temperature range (attributed to either LDPE or LLDPE) were shifted to a higher melt temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1009–1016, 2000     

Interfacial phenomena in cellulose/polyethylene composites

A systematic study of the mechanical properties of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) filled with cellulose was conducted. When benzoyl peroxide (BPO) was used and when the components were mixed at 160°, the yield strength of the LLDPE/cellulose composite increased by 70% whereas that of HDPE increased by only 15%. If BPO is replaced by dicumyl peroxide (DCP) the yield strength attains a plateau value at a lower concetration. Yield strength for the cellulose/LLDPE system, where the cellulose had been pretreated with silane, showed a relatively small improvement as compared to the effect of the peroxides addition. The presence of gel in the compound is also discussed.     

Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

The effect of chemical crosslinking on the properties of Rotomolded high density polyethylene

In this study, high density polyethylene (HDPE) was combined with dicumyl peroxide (DCP) to produce crosslinked parts via rotational molding. The effect of DCP content (0.1–2.5 phr) on the crosslinking degree was investigated to determine its effect on the chemical, mechanical, physical, and thermal properties of HDPE. The gel content and crosslink density was found to increase with DCP content. These trends led to a reduction in the degree of crystallinity, melting, and crystallization temperature. Thermogravimetric analysis (TGA) showed that crosslinked HDPE (xHDPE) has higher thermal stability than the neat matrix in both air and nitrogen atmosphere. In addition, a direct relationship was observed between improved thermal resistance and higher impact strength. Finally, relationships between the tensile properties of xHDPE and the degree of crystallinity were observed, which were all controlled by the level of crosslinking. These results have the potential to advance the manufacturing of high performance materials suitable for a wide range of applications such as automotive parts, agricultural products, chemical storage tanks, large waste containers, and fuel tanks in general.